Dog - just plain white krylon primer. There are some modeling types out there but I kept forgettting to buy them so tried the Krylon and am happy with the results. (you can get it at Wal Mart etc...) I just spray it out of the can. Again, just a nice light coat to cover the model. You don't want to make it white, just enough to give the paint something to hang on to. (it will look a little splotchy and the plastic shows through)
as far as weathering AFTER the paint this site by a member here, Swanny says it all!
http://www.swannysmodels.com/TheCompleteFuture.html
http://www.swannysmodels.com/Weathering.html
You should either future or gloss coat your model prior the decals, then wash and weather it. then future again and dull coat. (the same Future Floor polish you find at the grocery store)
Are you airbrushing these paints or hand brushing?